Hereinafter, general cyclic prefix (CP) used during data symbol transmission will be described in brief.
FIG. 1 is a diagram illustrating an example of a symbol structure which includes a cyclic prefix symbol.
A cyclic prefix symbol is used to prevent inter symbol interference (ISI) from occurring and prevent inter subcarrier orthogonality from being destroyed in an orthogonal frequency division multiplexing (OFDM) mode which is a multiple carrier modulation mode.
For example, while each symbol is being transmitted through a multi-path channel, a guard interval longer than delay spread of a channel is inserted between continuous symbols. At this time, to prevent inter subcarrier orthogonality from being destroyed, a signal of the last interval (i.e., guard interval) of an effective symbol interval is copied to be inserted into the front of the symbol. This will be referred to as a cyclic prefix symbol.
Referring to FIG. 1, a symbol period (Ts) becomes a sum of an effective symbol interval (Tb) and a guard interval (Tg). A receiving end performs demodulation by selecting data corresponding to the effective symbol interval after removing the guard interval. A transmitter and the receiving end can be synchronized with each other using the cyclic prefix symbol, and can maintain orthogonality between data symbols.
Currently, a wireless access system is not equally used for all communication environments. There are one or more providers which provide respective wireless access systems. Different cyclic prefix symbol lengths may be used by the respective wireless access systems. Also, with the development of technology, a wireless access system provided to a single provider can be designed to have a length different from that of the existing cyclic prefix symbol.
A legacy system means a system conventionally defined. An evolved system means a system evolved from the legacy system or a system newly defined.
The evolved system can be divided into a legacy support mode and a legacy disable mode, wherein the legacy support mode supports the legacy system and the legacy disable mode supports the evolved system only.
The legacy support mode means a mode supported by the evolved system for compatibility with the existing wireless access system. Namely, the legacy support mode means that the evolved system supports the legacy system in transmission and reception between the evolved system and the legacy system. The legacy support mode can be divided into two types.
One type of the legacy support mode means that a legacy base station and a legacy mobile station can communicate with each other without being affected by the evolved system. Also, one type of the legacy support mode means a status where the legacy base station and the legacy mobile station can communicate with each other.
The other type of the legacy support mode means a status where a base station that can support both the legacy system and the evolved system can perform data communication with both the legacy mobile station and an evolved mobile station.
However, a problem may occur in the legacy support mode when different cyclic prefix lengths are used for the legacy system and the evolved system. For example, the evolved mobile station does not know whether a current mode is a legacy support mode or a legacy disable mode after detecting an evolved preamble. Namely, since the evolved mobile station cannot know CP length information, the evolved mobile station fails to exactly detect a control channel or a data channel.